The present invention relates to a thermal printer for printing an image or characters on a print paper by transferring ink of an ink film through heating. Generally, the ink film includes a base film on which an ink layer of several microns thick is formed.
A conventional thermal printer of this kind comprises a thermal head having a plurality of heating elements and a platen opposite to the head. An inked film and print paper piled on the inked film are inserted between the thermal head and the platen together. Then, the inked film and the print paper are feed to the position where the thermal head is provided to be performed a transfer operation. The inked film and the paper are fed simultaneously at the same speed in the same direction. That is, feeding mechanisms for the inked film and the paper are made to interlock with each other at all times.
The paper feeding mechanism is driven while the paper is inserted to the thermal printer, performed the transfer operation, and run out of the printer. That is, the paper is fed by the paper feeding mechanism from its insertion to extraction to the thermal printer. Accordingly, the paper can be set to and extracted from the printer, automatically.
However, the inked film is fed unavoidably by the inked film feeding mechanism while the transfer operation is not be carried out since the mechanism is always in mechanical contact with the paper feeding mechanism. Therefore, unused portion is left on the inked film where an ink layer is still remain on a base film. Further, even if the inked film is used for the transfer operation, there arises unused portion on the inked film where no image and character is to be printed. There may be the case where such unused portion of the inked film amounts to 50% or the more. Such wastefulness of the inked film cannot be neglected in view of the expensiveness of the inked film.
Further, in the conventional thermal printer, the inked film feeding mechanism and the thermal head are provided on a printer body, and the paper feeding mechanism and a platen roller are provided on a printer cover for covering the upper surface of the printer body. Consequently, the platen roller is located opposite to the upper surface of the thermal head.
The inked film and the paper are wound around the circumference of the platen roller about half turn. Printing operation is carried out on the circumferential surface of the platen around where the inked film and the paper are wound by the thermal head. A "center type" thermal head is employed which has a plurality of heating elements arrayed in a shallow groove at the central portion.
However, since the paper is wound around the platen roller, the printer cannot print the image and characters on a stiff thick paper. To this end, if the printer is constructed such that the paper is to be fed flatly, the paper may be fed with its printed surface downward, and thus, printed characters are not visible instantly. Further, since the thermal head is provided with its transfer surface facing upward, dust is easy to stick on the heating element so as to make the printing quality poor.
On the other hand, this type of printer has an internal memory for storing character patterns corresponding to the character codes so as to print the characters instructed by a host device. Besides, a multitude of fonts are present for each character, such as italic type, print hand type, and the like. However, it is impossible to store all the fonts in the internal memory because it requires a vast memory capacity and brings problems in mounting and price.
Thus, it is proposed that framing the memory for character patterns to a cassette ready for replacing by operators,in which character patterns for one font are stored in one cassette. Then, characters of many types of fonts can be printed on one printer by replacing the cassette. Further, since the operator may prepare the memory cassettes only the necessary font to use, substantial memory capacity can be minimized in the printer.
A conventional structure for detaching the memory cassette in this type to the dot printer is shown in FIG. 1. A connector 2 is provided on a external surface of a printer 1 for engaging a connector 4 projecting from a memory cassette 3. The memory cassette 3 is prepared in variety to cope with each font, and thus, the printer 1 is capable of printing a variety of fonts by replacing the memory cassette 3.
The connector 4 of the memory cassette 3 has a plurality of terminals for address bus, data bus, power supply and earth. These terminals come in contact with reception terminals of the connector 2 to supply character patterns with the printer 1. Reversaly, the printer 1 supplies electrical power with the Read Only Memory (ROM) in the memory cassette 3 via one of the terminals. The printer 1 sends character codes to the memory cassette 3 via the terminals for address while the memory cassette 3 sends instructed character patterns to the printer 1 through the terminals for data.
In this apparatus, a voltage is always applied to the power terminal of the connector 2 even when the memory cassette 3 is not installed. Accordingly, a voltage is suddenly supplied to the power terminal of the connector 4 when the memory cassette 3 is inserted, and thus, a large current flows in ROM in the memory cassette 3 to break down contents of the memory. Particularly, the memory break is often occurred when the power is supplied before earth terminals of the connector come in contact completely with each other.
To cope with such defection, it can be considered that making a main power supply of the printer 1 be cut off when the memory cassette 3 is replaced. However, in this case, contents of another internal memory (voltatile memory for storing a job program, or the like) of the printer 1 is erased at the same time, thereby, an extra operational would be required for restoration.